Freestanding films with electric field-enhanced piezoelectric  coefficients

ABSTRACT

A method to produce low-temperature sinterable powders which are then subsequently used to fabricate freestanding piezoelectric films with very large electric-field-enhanced piezoelectric response is provided. The −d 31  coefficient for PMN-PT layers can be as high as 2000 pm/V, larger than that of commercial single crystalline PMN-PT bulk materials, at 10 kV/cm (or 20 V over the 20-micron film thickness). In contrast to single crystals, the polycrystalline freestanding films are easy to fabricate and can be made into any size. The films are also easily miniaturized. The method can be applied to nearly any piezoelectric material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/392,116 filed on Mar. 29, 2006, currently pending, which, in turn isa non-provisional of U.S. Provisional patent application No. 60/666,036,filed on Mar. 29, 2005, now expired.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of piezoelectric ceramics. Inparticular the invention relates to polycrystalline ceramics with highpiezoelectric coefficients in thin-layer geometry.

2. Description of the Related Technology

Piezoelectric ceramics such as quartz and lead zirconate titanate (PZT)are the primary component in most actuator applications, which command amulti-billion dollar annual market. PZT dominates the current actuatormarket because of its high piezoelectric coefficients with d₃₃ rangingfrom 100 to 700 and d₃₁ ranging from −50 to −300 pm/V. The coefficientsd₃₃ and d₃₁ measure the ratio of the strain parallel and perpendicularto the direction of the electric field, respectively. In general, themagnitude of d₃₃ is roughly twice that of d₃₁. For comparison, the −d₃₁of quartz is less than 10 pm/V.

However, even with such high piezoelectric coefficients, for a typical 1mm thick plate, the strains generated by 1000 V are still less than 0.1%in PZT's. The market demand for high-strain actuators has fueled intenseresearch interest in developing piezoelectrics with high piezoelectriccoefficients (that is, higher than those of commercial PZT)

For polycrystalline piezoelectric ceramics, including PZT, to be useful,they must be polarized in order to have high piezoelectric coefficients.Before polarization, the orientations of domains are random with no netpolarization. After polarization, many domains are aligned or switchedto the direction of the applied electric field resulting in a finitepolarization. However, the domains in polycrystalline materials are notas easily aligned as in a single crystal.

The piezoelectric behavior of a polarized polycrystalline material underan electric field comes from three effects: the intrinsic piezoelectriceffect, the domain wall motion, and the electrostrictive effect. Theintrinsic piezoelectric effect is related to the deformation of thelattice structure by the applied electric field. The intrinsicpiezoelectric effect is generally small. The electrostrictivedeformation is proportional to the square of the applied electric fieldand is also generally small. The main effect produced by the electricfield comes from the domain wall motion. When the domain walls moveunder an electric field, i.e., domain switching, the net polarization ofthe sample changes thereby resulting in deformation of the material.Only non-180° domain switching causes dimensional changes, whereas 180°domain switching does not. Domain wall motion is known to be influencedby point defects, grain boundaries, microstructures, and compositions.

Due to the demand for increasingly smaller actuators and devices, mucheffort has been devoted to developing thin-film-based microactuators andmicrosensors. Most of the piezoelectric thin films investigated weregrown on a silicon-based substrate for integration with the siliconcircuitry. However, after more than one decade of development, thinfilms generally exhibited a smaller piezoelectric coefficient than thebulk material due to substrate pinning that seriously hindersdomain-wall motion in the film geometry. For example, bulk leadzirconate titanate (PZT) has a d₃₃ of about 500 pm/V, while PZT thinfilms exhibit a d₃₃ of about 100-200 pm/V.^(1,2,3) The lowerpiezoelectric coefficient in thin films is generally attributed to theclamping effect of the substrate.

Recently, a major breakthrough for high-strain piezoelectric ceramicswas the development of single crystalline piezoelectric materials. Forexample, specially cut (001) lead zirconate niobate-lead titanate(PZN-PT) single crystals have a d₃₃ of 2500 pm/V.⁴ In comparison, PMN-PTbulk ceramics have a d₃₃ about 720 pm/V.^(5,6) (010)-cut PMN-PT singlecrystals have a d₃₃ greater than 2000 pm/V and a d₃₁ of −930 pm/V.⁷PZN-PT single crystal materials have a d₃₃ on the order of 2000 pm/V,significantly higher than that of its polycrystalline counterpart. Thisis because the domains in a single crystal can be more easily aligneddue to the transformation from a rhombohedral to a tetragonal structurewith application of a sufficiently large electric field.

Even though single crystal piezoelectric materials have highpiezoelectric coefficients, they are difficult to process. Specializedgrowth methods have to be designed and the size of the crystals islimited. Furthermore, only a small fraction of piezoelectric materialscan be grown into a single crystal. For example, the most popularpiezoelectric, PZT, cannot currently be grown into a single crystal. Dueto the scarcity of single crystal piezoelectric materials, their priceis very high as well. Furthermore, single crystal materials aremacroscopic in size. They are difficult to miniaturize for many MEMS(microelectro-mechanical systems) applications.

Therefore, there exists a need for providing polycrystalline ceramicswith high piezoelectric coefficients in thin-layer geometry.

SUMMARY OF THE INVENTION

Accordingly, it is an object of certain embodiments of the invention toprovide polycrystalline ceramics with high piezoelectric coefficients ina thin-layer geometry. Other embodiments of the invention providedielectric/ferroelectric ceramics.

One embodiment of the invention relates to a precursor suspensioncoating (PSC) method for fabricating low-temperature, sinterablepolycrystalline ceramics. In this method, submicron crystalline powderwas first obtained by dispersing coated crystalline particles in asolution reactive with the coating followed by calcination. The calcinedpowder was subsequently suspended in a precursor coating solution toform a precursor powder that could be sintered at a temperature at orbelow about 900° C. The low sintering temperature may be due to thereactive sintering of the precursor powder during the calcination step.

In another embodiment, the present invention relates to a tape castingmethod. A mixture of polycrystalline powder, a dispersing resin and asolvent, is prepared. The mixture is mixed to form a slurry. The slurryis then tape cast into polycrystalline layers.

In another embodiment the present invention relates to polycrystallinepiezoelectric/dielectric/ferroelectric materials. In another embodimentthe present invention relates to polycrystallinepiezoelectric/dielectric/ferroelectric materials made by one or more ofthe methods of the present invention.

These and various other advantages and features of novelty thatcharacterize the invention are pointed out with particularity in theclaims annexed hereto and forming a part hereof. However, for a betterunderstanding of the invention, its advantages, and the objects obtainedby its use, reference should be made to the drawings which form afurther part hereof, and to the accompanying descriptive matter, inwhich there is illustrated and described a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a photograph of a translucent PMN_(0.35)-PT_(0.35) tape 22≧m thick and 1 cm in diameter.

FIG. 2 shows a SEM micrograph of a 22 ≧m thick PMN_(0.65)-PT_(0.35)tape.

FIG. 3 shows the field enhancement of piezoelectric coefficient d₃₁ infreestanding PMN-PT tape (filled triangles).

FIG. 4 shows the piezoelectric coefficient d₃₁ of a freestanding PMN-PTtape measured by lateral displacement of a PMN-PT rectangular stripunder DC electric fields.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Using a tape casting method, freestanding polycrystalline films ofPMN-PT that have a d₃₁ of about −2000 pm/V at E=9 kV/cm, higher thanthat of commercial PMN-PT single crystals, were produced. As a result,use of the tape casting fabrication method has solved the problem ofproducing piezoelectric ceramics with high piezoelectric coefficients.Freestanding films avoid many problems of thin films on a substrate suchas interfacial reactions, thermal expansion coefficient mismatch,substrate pinning/clamping effects, etc.

The highly piezoelectric thin layers have many applications in, forexample, sensors, actuators and MEMS. The layers can be stacked to forma multi-layer actuator that can generate large and precise displacementsand forces with a relatively small voltage, e.g., <20V. The thin layergeometry allows easy miniaturization by simple wire-saw cutting toproduce high energy density, but low-power consumption, MEMS devices.Each of the actuator, sensor, and MEMS areas commands a large market.

In addition, one embodiment of the invention relates to a precursorsuspension coating (PSC) method for fabricating low-temperature,sinterable polycrystalline[Pb(Mg_(1/3)Nb_(2/3))O₃]_(0.63)-[PbTiO3]_(0.37) (PMN-PT) ceramics. Inthis method, submicron crystalline PMN powder was first obtained bydispersing Mg(OH)₂-coated Nb₂O₅ particles in a lead acetate/ethyleneglycol solution followed by calcination at about 800° C. The crystallinePMN powder was subsequently suspended in a PT precursor solutioncontaining lead acetate and titanium isopropoxide in ethylene glycol toform the PMN-PT precursor powder that could be sintered at a temperatureas low as about 900° C. The combination of using sinterable PMN powdersand the elimination of defects created by placement of thin films onsubstrates, produced superior PMN-PT films.

The low sintering temperature may be due to the reactive sintering ofthe PMN-PT precursor powder. The reaction between the PT and the PMNoccurred in the same temperature range as the densification process,850-1000° C., thus significantly accelerated the sintering process. Thepresent PSC technique is robust and can be readily applicable to otherlead based piezoelectrics and other non-lead-based materials as well,and may be applicable to the use of other suitable coating materials.

Example for Low-Temperature, Sinterable Powders

The synthesis of the reactive PMN-PT precursor powder entailed twoprecursor suspension coating (PSC) steps. The first PSC step involvedsuspending Mg(OH)₂-coated Nb₂O₅ particles in a lead acetate/ethyleneglycol solution to obtain the PMN precursor powder. The second PSC stepinvolved suspending the calcined PMN powder in a PT precursor containinglead acetate and titanium isopropoxide solution in ethylene glycol.

Niobium oxide (Nb₂O₅, 99.9%, Aldrich Chemical Company, Inc., Milwaukee,Wis.) was ultrasonically dispersed in distilled water after addingammonium hydroxide (4.96 N solution in water, Aldrich Chemical Company,Inc., Milwaukee, Wis.). Magnesium nitrate solutions were first preparedby dissolving magnesium nitrate hexahydrate (Mg(NO₃)₂I6H₂O, 99%, AldrichChemical Company, Inc., Milwaukee, Wis.) in distilled water. Themagnesium nitrate solution was dropped into the niobium oxidedispersion. Since the pH of the dispersion was between 10 and 11,magnesium hydroxide precipitated and coated on the niobium oxideparticles. The final concentration of the Nb₂O₅ powder in the coatingsuspension was 26.6 g/L or 0.1 M.

The Mg(OH)₂-coated Nb₂O₅ particles were dried subsequently at 150° C. ona hot plate.^(8,9) After drying, the Mg(OH)₂-coated Nb₂O₅ powder wasadded to a lead precursor solution where lead acetate anhydrous(Pb(CH₃COO)₂.2Pb(OH)₂, Fluka) was dissolved in ethylene glycol(HOCH₂CH₂OH, Alfa Aesar) with 15% excess lead. These procedures completethe first PSC step.

The slurry was dried at 150° C. on a hot plate. Pyrochlore-freeperovskite PMN powder was obtained by first heating the PMN precursorpowder at 1° C./min to 300° C. for 2 hr followed by 5° C./min to 800° C.for 2 hr.¹⁰

The perovskite PMN powder was then suspended in a PT precursor solutioncontaining lead acetate and titanium isopropoxide (Ti(OCH(CH₃)₂)₄, AlfaAesar) in ethylene glycol (EG) and ball milled for 24 hr. The finalnominal composition was PMN_(0.63)-PT_(0.37)with 10% lead excess. Theball milled PMN-PT precursor slurry was then dried at 200° C. on a hotplate for 2 hr and heated at 1° C./min to 300° C. for 2 hr. After dryingat 300° C., low-temperature sinterable PMN-PT powder was obtained and ina form ready for tape casting.

Example of Tape Casting

Tape-casting is a forming technique used to produce thin ceramic (andmetallic) layers which are formed on a carrier film by the shearingaction of a doctor blade on a moving ceramic slurry. The tape contains abinder system, which serves as a carrier for the ceramic powders; i.e.,it holds up the ceramic powders so that they will sinter after thebinder is burned out. Some of the desirable properties of the bindersystem include clean decomposition, good solubility in a wide range ofsolvents and decent green-strength.

Acrylic resins, such as methyl methacrylate and ethyl methacrylate havelong been used in binder systems for ceramic tape casting for their lowash residue upon binder burnout. The use of longer chain alcohol (e.g.alcohols having 4-12 carbon atoms in the chain) esters of methacrylic oracrylic acid yielded much lower decomposition profiles and improvedelectrical properties. For example, iso-butyl methacrylate, having amolecular weight of 60,000 (Rohm & Haas, Paraloid B-38), yieldsvirtually no measurable ash content upon decomposition. This is theresult of the use of acrylic acid ester or methacrylic acid estermonomers of 4 carbon atoms or higher, and also includes pentyl acrylate,pentyl methacrylate, hexyl acrylate, hexyl methacrylate, heptylacrylate, heptyl methacrylate, octyl acrylate, octyl methacrylate, nonylacrylate, nonyl methacrylate, decyl acrylate, decyl methacrylate,undecyl acrylate, undecyl methacrylate, dodecyl acrylate, and dodecylmethacrylate.

Dispersion of the PMN-PT powders is accomplished by a blend of low(about 1,000-10,000) and mid (about 40,000-80,000) molecular weightdispersing resins. A dispersing resin from Rohm and Haas, ParaloidDM-55, with a molecular weight of 6,000, is a blend of methylmethacrylate, iso-bornyl methacrylate, and proprietary monomers inamounts of 0.1% by weight to 30% by weight, based upon the weight of theceramic powders. This is blended with an iso-butyl methacrylate polymer,with a molecular weight of ˜60,000 (Paraloid B38 from Rohm and HaasCompany) in an amount of 0.1% to 50.0% by weight, based upon weight ofthe total binder. This dispersion method allows for a higher powderloading, resulting in a denser final part with less shrinkage.

A mill jar was charged with yttria stabilized zirconia media, PMN-PTpowder, a dispersing resin blend, and a solvent blend of an alcohol anda ketone; for instance, isopropyl alcohol and methyl ethyl ketone, butcould also include various alcohols, ketones, esters, glycol ethers,aliphatic hydrocarbons, and aromatic hydrocarbons. The mill base wasball milled for 16-24 hrs. Then, the balance of the binder/resin wasadded (1%-50% by weight, based on weight of the ceramic powder), alongwith a phthalate-based plasticizer (binder:plasticizer ratio of about2:1 to 3:1); the mixture was then rolled for an additional 24 hrs. Theslurry was then de-aerated, and cast into various thicknesses. The greentapes were then punched and prepared for binder removal and sintering.

Examples of Electric-Field Enhanced Freestanding Films

FIG. 1 shows a photograph of a translucent PMN-PT film made by thepresent process. FIG. 2 shows an SEM image of the cross-section of aPMN-PT film. Clearly, the film is fully dense with a uniform grain sizedistribution. FIG. 3 shows the piezoelectric coefficient d₃₁ of a 22 ≧mfilm versus the electric field where the d₃₁ was deduced from thedisplacement of a 2.5 mm long cantilever consisting of a PMN-PT layerbonded to a 5-≧m thick copper layer. As can be seen, the −d₃₁coefficient increased to 2000 pm/V at about 9 kV/cm. The present −d₃₁value of 2000 pm/V was much higher than that of commercial PMN-PT singlecrystals.

To confirm the enhanced −d₃₁ coefficient, direct measurement of thelateral elongation of a freestanding film was performed when an electricfield was applied perpendicular to the film. The results of the directmeasurement are shown in FIG. 4. The direct lateral elongationmeasurements also showed an enhanced −d₃₁ of 2000 pm/V at around 10kV/cm, consistent with the data shown in FIG. 3 indicating that theenhanced d₃₁ coefficient is an intrinsic effect of the freestanding filmand not an effect of the bi-layer structure used in the cantileverdisplacement measurement described above.

Thus, in another aspect, the present invention relates topolycrystalline ceramics having a −d₃₁ of at least 1500 pm/V at around10 kV/cm, more preferably a −d₃₁ of at least 1800 pm/V at around 10kV/cm, and, most preferably, a −d₃₁ of at least 2000 pm/V at around 10kV/cm.

Nonlinear piezoelectric responses of piezoelectric materials have beenobserved in several studies. For example, in soft PZT bulk ceramics,−d₃₁ was observed to increase about 50% from 170 pm/V at low fields(<0.1 kV/cm) to about 230 pm/V at higher fields (˜1 kV/cm).^(11,12)However, −d₃₁ values at even higher fields (5˜20 kV/cm) were notavailable because such high fields were limited by the high voltagesneeded across the thickness (˜0.5 mm)

In another example, a series of commercial soft PZT ceramics showedfield enhancement increases of d₃₃ from 400 pm/V at below 0.1 kV/cm to1600 pm/V at 6 kV/cm.¹³ For relaxor PMN-PT bulk ceramics, it wasreported that field enhancement increases of d₃₃, from 100 pm/V to 1200pm/V were achieved when the field was increased to 5 kV/cm, above whichd₃₃ quickly degenerated.¹⁴

In PZN-PT single crystals, however, field enhancement increases of d₃₃from 2000 pm/V to 5000 pm/V with linear-nonlinear threshold at 2˜4 kV/cmwere achieved,¹⁵ which is comparable to that of the freestanding PMN-PTtapes (3˜5 kV/cm).

The disadvantages of PZN-PT or PMN-PT single crystals are thedifficulties in processing and machining. One of the approaches is toproduce single crystals by the flux method¹⁶ where the ingredientpowders are dry mixed and then loaded into a platinum crucible and heldat the soaking temperatures ˜1100-1200° C., followed by slow cooling˜1-5° C./h. About two thirds (⅔) of the initial mixture are excess leadwhich is needed to generate a liquid phase (flux) and to counter theeffect of lead evaporation at the high soaking temperature. The crystalsobtained are rather small (3 to 20 mm), and they need to be recovered byhot HNO₃ to separate the crystals out of the rest of the melt. Also, thecrystal growing process is slow (>100 hours).

A vertical Bridgmen-Stockbarger method is another approach to makePMN-PT single crystals.¹⁷ The starting materials are high purity PbO,MgO, Nb₂O₅ and TiO₂. After mixing, they are loaded into a platinumcrucible and heated to a maximum temperature of 1395° C. with a gradientin the crystal growing direction. With a seed crystal, the growth ratewas controlled at 0.4˜0.8 mm/hr at a temperature gradient of 20° C./cm.Typical crystal size is about 1 cm. Although stoichiometric ingredientswere used, the temperature was much higher than that of the flux method.The crucible must be tightly sealed to prevent lead evaporation.

One advantage of the process of the present invention is that thedefects associated with a substrate that may prevent domain wall motionare eliminated by the freestanding layer geometry. The highpiezoelectric coefficient is found in bulk samples under the sameelectric fields. The electric-field enhancement effect on thepiezoelectric coefficient only occurs in the freestanding thin layergeometry. Conceivably, the thin layer, which has only a few grainsacross the thickness, allows the applied electric field to penetratethrough the sample more easily, thus permitting the domain wall motionto be activated in a more complete manner

Various embodiments of the present invention may offer one or more ofthe following advantages:

1. One embodiment of the present invention provides a precursorsuspension coating (PSC) method to produce reactive powders withsintering temperatures lower than 1000° C.2. Another embodiment of the invention provides a tape casting methodwith minimal burnout product which can be used to produce freestandingfilms using the low-temperature sinterable powders.3. The method of the present invention is applicable to mostpiezoelectric/dielectric/ferroelectric ceramics including solid-solutionperovskites such as PMN-PT, PZT, and lead-freepiezoelectrics/dielectrics/ferroelectrics.4. The method of the present invention applies to PMN-PT powders madefrom Mg(OH)₂-coated Nb₂O₅ powders.5. The method of the present invention applies to PZT powders made fromZrO₂-coated TiO₂ powders.6. The PMN powders produced from Mg(OH)₂-coated Nb₂O₅ powders coatedwith the PT precursor can be sintered at temperatures below 900° C., andas low as 850° C.7. The freestanding layers have a very large electric-field enhancedpiezoelectric coefficient for a layer thickness of <50 ≧m.8. The binder system may be an acrylic acid ester or methacrylic acidester of 4 carbons or higher.9. The dispersant may be a blend of low and mid molecular weightdispersing resins.

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

REFERENCES

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1. A polycrystalline ceramic material having −d₃₁ of at least 1500 pm/Vat around 10 kV/cm.
 2. A polycrystalline ceramic material as claimed inclaim 1, having a −d₃₁ of at least 1800 pm/V at around 10 kV/cm.
 3. Apolycrystalline ceramic material as claimed in claim 1, wherein thematerial is a freestanding film.
 4. A polycrystalline ceramic materialas claimed in claim 2, wherein the material is a freestanding film.
 5. Aproduct as claimed in claim 4, wherein the polycrystalline ceramicmaterial is formed by reactive sintering of a calcined product whichcomprises lead magnesium niobate and a material reactive with thecalcined product which comprises lead titanate.
 6. A polycrystallineceramic material made by a method comprising the steps of: dispersingcoated particles in a solution of a material reactive with the coatingof said coated particles to form a dispersion, drying the dispersion toform a powder; calcining the powder to form a calcined product whereinthe coating of the coated particles has at least partially reacted,suspending the calcined product in a precursor solution for a materialreactive with the calcined product, forming a powder of the calcinedproduct and the material reactive with the calcined product from saidprecursor solution, tape casting a slurry of said powder of the calcinedproduct and the material reactive with the calcined product, a binder, adispersing resin and a solvent, separating the layer of the calcinedproduct and the material reactive with the calcined product from thecarrier tape to provide a freestanding film of the calcined product andthe material reactive with the calcined product, and reactive sinteringthe freestanding film of the calcined product and the material reactivewith the calcined product, under conditions whereby the calcined productreacts with the material reactive with the calcined product to provide asintered freestanding polycrystalline ceramic film.
 7. A product asclaimed in claim 6, wherein said coated particles are selected from thegroup consisting of: Mg(OH)₂-coated Nb₂O₅ powders and ZrO₂-coated TiO₂powders.
 8. A product as claimed in claim 7, wherein said powder of thecalcined product and the material reactive with the calcined product isselected from the group consisting of: lead-freepiezoelectrics/dielectrics/ferroelectrics.
 9. A product as claimed inclaim 8, wherein the coated particle is a Mg(OH)₂-coated Nb₂O₅ powderand the material reactive with the coated particles comprises lead. 10.A product as claimed in claim 9, wherein the material reactive with thecalcined product comprises lead titanate.
 11. A product as claimed inclaim 10, wherein the sintering step is carried out at a temperaturebelow about 900 degrees Celsius.
 12. A product as claimed in claim 11,wherein the precursor comprises lead acetate and titanium isopropoxide.13. A product as claimed in claim 6, wherein the binder is selected fromthe group consisting of acrylic resins.
 14. A product as claimed inclaim 13, wherein the binder is selected from the group consisting of:acrylic acid esters and methacrylic acid esters made from monomerscontaining 4-12 carbon atoms.
 15. A product claimed in claim 14, whereinthe binder is selected from the group consisting of: methylmethacrylate, ethyl methacrylate, iso-butyl methacrylate, pentylacrylate, pentyl methacrylate, hexyl acrylate, hexyl methacrylate,heptyl acrylate, heptyl methacrylate, octyl acrylate, octylmethacrylate, nonyl acrylate, nonyl methacrylate, decyl acrylate, decylmethacrylate, undecyl acrylate, undecyl methacrylate, dodecyl acrylate,and dodecyl methacrylate.
 16. A product as claimed in claim 13, whereinthe dispersing resin is a blend of a low molecular weight dispersingresin having a molecular weight in the range of about 1,000-10,000, anda high molecular weight dispersing resin having a molecular weight inthe range of about 40,000-80,000.
 17. A product as claimed in claim 16,wherein the low molecular weight dispersing resin comprises methylmethacrylate and iso-bornyl methacrylate.'
 18. A product as claimed inclaim 17, wherein the high molecular weight dispersing resin comprisesiso-butyl methacrylate polymer.
 19. A product as claimed in claim 6,wherein the calcined product comprises lead magnesium niobate and thematerial reactive with the calcined product comprises lead titanate.